Metal-Oxide Semiconductor Field-Effect Transistors (MOSFETs) have been scaled down in size in order to improve their cost and performance. Minimum feature sizes have been reduced to dimensions that are now less than 100 nanometers (nm). When channel lengths are reduced to these levels, leakage currents such as off-state leakage currents can be significantly increased.
One approach that has been used to overcome the problems associated with short channel lengths is to fabricate the MOSFETs on Silicon On Insulator (SOI) wafers. Types of SOI wafers can include, for example, Separation by IMplantation of Oxygen (SIMOX) wafers. When MOSFETs are fabricated on SOI wafers, off-state leakage currents are reduced. For example, leakage currents that result from drain-induced barrier lowering are reduced because the buried oxide layer can block the lateral penetration of the drain induced electric field. One type of MOSFET that can be fabricated on SOI wafers is a multi-gate field effect transistor (MuGFET) that uses more than one gate. FinFETs, for example, are double-gate devices in which silicon is etched into a fin-shaped structure and the gate is formed around and over the fin. Another type of MOSFET that can be fabricated on SOI wafers is an Ultra-Thin Body (UTB) MOSFET that operates in a fully depleted mode. Other approaches that have been used are to fabricate the MOSFETs on Silicon Germanium (SiGe) on Insulator (SGOI) substrates or on Ge on Insulator (GOI) substrates. Both SGOI MOSFETs and GOI MOSFETs have shown significant carrier mobility enhancement and improved scaling properties compared to MOSFETs fabricated on Si wafers.
One problem that MOSFETs fabricated on SOI, SGOI or GOI substrates have is self-heating. Self-heating is caused by a conversion of electrical energy into thermal energy that results in increased lattice temperatures, degraded electron mobility, and reduced transconductance and channel current. Self-heating results because of the difference between the thermal conductivities of silicon dioxide (SiO2), SiGe, Ge and Si. The thermal conductivity of Si is 1.5 W/cm-° C. which is greater than the thermal conductivity of SiO2 (0.014 W/cm-° C.), the thermal conductivity of Si0.75Ge0.25 (0.085 W/cm-° C.) or the thermal conductivity of Ge (0.6 W/cm-° C.)